Ignition control for piezoelectric ignition system



Jan. 11, 1966 J. H. FREEMAN, JR

IGNITION CONTROL FOR PIEZOEL-ECTRIC IGNITION SYSTEM Filed Nov. 20, 1961 2 Sheets-Sheet 1 '0 mallllik INVENTOR.

JOHN H. FREEMAN, JR. BY Zflvwj ,A 042 ATTORNEY Jan. 11, 1966 J. H. FREEMAN, JR 3,229,153

IGNITION CONTROL FOR PIEZOEL-ECTRIC IGNITION SYSTEM ATTORNEY United States Patent 3,229,153 IGNITION CONTRO-L FOR PIEZOELECTRIC IGNITION SYSTEM John H. Freeman, Jr., Crystal Lake, Ill., assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Filed Nov. 20, 1961, Ser. No. 153,311 12 Claims. (Cl. 315-55) This apparatus relates to ignition systems for internalcombustion engines in which the high-tension voltage required to fire a spark plug is produced by the alternate compression and relaxation of a piezoelectric element, and,-m0re particularly, to a piezoelectric ignition system in which high-tension voltage is applied to the spark plug at the proper time by the incorporation of a gap in the high-tension circuit which operatively connects the piezoelectric element and the spark gap.

Until recently, sources of high-tension voltage required to fire spark plugs in internal-combusition engines have been either battery-energized ignition coils or magnetos. Those who are familiar with ignition systems utilizing ignition cells and magnetos are aware of the disadvantages inherent in these ignition systems, such as battery failure in ignition systems utilizing ignition coils and low-voltage output at cranking speeds where magnetos are utilized. In recent years piezoelectric ignition systems have been used as improved high-tension voltage sources forsparkignited internal-combustion engines. Piezoelectric ignition systems have greater simplicity, are less expensive, and are easier to maintain than conventional electromagnetic systems. In addition, they provide higher available voltages at comparable size, provide peak available voltage at cranking speed, are independent ofbattery condition and voltage drop caused by starter load, and experience no drop in available voltage with increasing engine speed. While their use has been limited thus far to small internal-combustion engines of thesinglecylinder type, it appears that in the future piezoelectric ignition systems may be utilized in larger internal-combustion engines.

In a piezoelectric ignition system, a piezoelectric generator, electrically connected in series with the spark gap of a spark plug, is alternately compressed and relaxed in timed relation to the engine cycle by a suitable motiontransmitting means operated by the engine crankshaft or camshaft. At the maximum value of its compression, a peak voltage of one sign appears across the terminals of the crystal, while, after discharge, a peak voltage of opposite sign appears at the maximum value of its relaxation. The voltages thus produced are of sufficient magnitude to fire the spark plug. Polycrystalline piezoelectric elements are preferably utilized in ignition systems since monocrystalline piezoelectric crystals, such as Rochelle salt, quartz, etc., have a tendency to failupon the application of the required mechanical stress to produce a sufiicient voltage for spark-plug firing. In general, the polycrystalline piezoelectric elements are composed of a polycrystalline piezoelectric material, such as a metallic titanate, or mixtures thereof combined with a ceramic binder, and fired. Examples of metals from which the polycrystalline metallic titanates are derived are barium, strontium, magnesium, manganese, lead and zirconium.

Piezoelectric ignition systems have been made less suscepti-ble to the condition of the spark plug, particularly with regard to shunt resistance, and the timing of the firing of the spark plug has been made variable by incorporating a switch in the high-tension wiring system between the piezoelectric element and the spark plug. In one form of such a switch, a stationary electrode operatively produced by the piezoelectric element.

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connected to the output or hot terminal of the piezoelectric element is in juxtaposition to a crankshaft-driven rotor disc, near the edge thereof. Axially in line with the stationary electrode, and separated therefrom by the rotor disc, is a second stationary electrode in juxaposition to the rotor disc, and electrically connected to the central electrode of the spark plug. At a predetermined time near one of the voltage peaks produced by the piezoelectric element, a circumferentially moving shunt or conductor on the rotor passes between the two stationary electrodes and closes the ignition circuit, thereby effecting firing of the spark plug. For application to four-stroke cycle engines, the rotor is provided with a second conductor, electrically grounded to the engine through the rotor shaft in order to discharge every otherpe-ak voltage Another form of a switch is disclosed in US. Patent 2,954,506. In view of the close tolerances necessitated in such a switch, it is costly and complex to maintain, difiicult to insulate, requires materials of exceptional dielectric strength, and is particularly vunerable to failure in the field due to dirt, oil, and/or high humidity.

This invention is based upon my discovery of a simpli-fied andimproved piezoelectric ignition system for use in spark-ignited internal combustion engines. In my invention, the .timing switch used in present conventional piezoelectric ignition systems, such as hereinbefore described, is replaced with a gap, either in the circuit which operatively connects the piezoelectric element and the spark plug, or in the spark plug itself. The gap avoids the aforementioned disadvantages of the switch but retains all of the advantages of a piezoelectric ignition system with a switch.

It is an object of this invention to provide an improved piezoelectric ignition system for internal-combustion engines.

Another object of this invention is to provide piezoelectric ignition system-s for preventing firing of the spark plug at undesired times during the engine cycle.

Another object of this invention is to provide a piezoelectric ignition system which assures accuratelyrepeatable firing of the spark plug.

A further object of this invention is to provide a piezoelectric ignition system in which an air gap is incorporated in the high-tension wiring *operatively connecting the piezoelectric element and the spark plug.

A still further object of this invention is to provide a piezoelectric ignition system in combination with aspark plug having a gap in the central electrode.

These and further objects of the invention will become apparent as the description proceeds and references made to the drawings in which:

FIGURE 1 is a circuit diagram of the piezoelectric ignition system of this invention, including a view in partial cross-section of a conventional mechanism for stress- 1 ing the piezoelectric element, in which the piezoelectric element is under maximum stress;

FIGURE 2 is a longitudinal cross-sectional view through the center of a spark plug which can be used in the piezoelectric ignition system of this invention;

FIGURE 3 is a schematic circuit diagram of apiezoeither in the high-tension wiring between the piezoelectric pick-up and the spark plug, or within the spark plug. Should the gap be located within the wiring, it is preferred that it be as close 'as possible to the spark plug to avoid inimical high-tension capacitance effects which would be associated with other locations, and that it be located within a housing of electrically non-conducting material. Means can be included to adjust the size of the gap, if so desired. 1 I

The size of the gap is adjusted so that the voltage which it permits to pass is always greater than that required to fire the spark plug. For example, if the required spark-plug voltage varies over a range of about 6,000 to 12,000 volts, depending upon variations in homogeneity of the fuel mixture, richness of the fuel mixture, engine load, engine speed, engine acceleration, spark-gap growth, and spark-gap erosion, the gap is adjusted to pass voltages greater than about 12,000 volts, and to prevent passage of lesser voltages.

For a more complete understanding of the piezoelectric ignition system of this invention, reference is made to FIGURE 1 which shows a conventional piezoelectric generator '8 in which the piezoelectric element is composed of a pair of cylindrical, polycrystalline, piezoelectric crystals Hand 12, made of barium titanate, lead zirconate titanate, etc., confined in end-to-end realtionship within the longitudinal cylindrical opening in holder 14 of an electrically non-conductive material. Suitable materials for holder 14 include ceramics, plastics such as fluoroethylenes, urea-formaldehyde resins, and the like. Po sitioned between piezoelectric crystals and 12, and in contact therewith, is cylindrical flexible conductor 16 which can take the form of a steel button coated with conducting rubber. Similar cylindrical flexible conductors 18 and 20 are provided at the remote ends of crystals 10 and 12, respectively. Holder 14 is held in place by any suitable method, as by strap 22 conforming to the shape of holder 14 and having outwardly extending feet 24 which are secured by bolts 26 to a portion of the engine 28, e.g., the crankcase.

Extending out of holder 14 and in contact with flexible conductor 18 is metallic bearing member 30. Stress is applied to the piezoelectric element through bearing member 30 by lever 32 which is pivotally mounted about pin 34 at the end remote from bearing member 30. Bearing against lever 32 is eccentric 36, keyed to shaft 38 which is driven by either the crankshaft or camshaft and rotated in timed relation to the engine cycle. The eccentric 36 is located on the side of lever 32 opposite that in contact with bearing member 30 between bearing member 30 and pin 34. Lever 32 is biased away from bearing member 30 by spring 40 which is held in place by bolt 42. The piezoelectric element is urged against lever 32 by setscrew 44 threaded through the wall of L-shaped bracket 46 and engaged against flexible conductor 20. L-shaped bracket 46 is secured to the engine by bolt 48. Setscrew 44 may be replaced with a selfadjusting load screw such as disclosed by E. Crankshaw and R. Arnold in A Piezoelectric Ignition System for Small Engines, at the 1961 S.A.E. summer meeting.

Electrically connected to flexible conductor 16 is piezoelectric output or hot terminal 50 on the outside wall of holder 14. The voltages produced by the piezoelectric elements are fed to spark plug 52 by high-tension wire 54 which is electrically connected to electrode 56, the piezoelectric pick-up, which is in juxtaposition to output terminal 50. The end of crystal 10 remote from terminal 50 is grounded by shaft 38 and spring 40 through lever 32, while .the remote end of crystal 12 is grounded through setscrew 44 and bracket 46. The circuit which operatively connects the output of the piezoelectric element and spark plug 52 in series is completed since the electrode of spark plug 52 which is not connected to lead 54 is grounded.

In the embodiment of the invention illustrated in FIG- URE 1, incorporated in high-tension lead wire 54 between spark plugs 52 and electrode 56 is gap 58, preferably about 0150-0188 inch, within housing 60 of an electrically non-conductive material. Since it may be desirable to vary gap 58, housing 60 includes plate 62, made of electrically conductive material, such as steel or brass, electrically connected to electrode 56 by lead 54 and having a threaded portion to match a threaded portion of movable electrode 64. Movable electrode 64 extends out of housing 60 and is provided with knurled thumb wheel 66 at the exposed end thereof. Movable electrode 64 can be moved toward and away from stationary electrode 68, electrically connected to the central electrode of spark plug 52, by turning thumb-wheel 66. Electrode 64 may be provided with a lo'cknut (not shown) to insure retention of it in any desired position and prevent vertical displacement due to engine vibration. A graduated scale correspondingto the space between electrodes 64 and 68 may be scribed on the portion of electrode 64 extending out of plate 62 so that a mark even with the top of plate 62 indicates the size of gap 58.

During the operation of the engine, shaft 38 rotates in timed relation to the engine cycle and causes lever 32 to rock back and forth about pin 34, applying increasing compressive force to crystals 10 and 12 during one-half of each revolution of shaft 38 and relaxing the compressive force during the other half revolution. During onehalf of each revolution of shaft 38 a voltage of one sign is produced by crystals 10 and 12, reaching a peak value when compression is at maximum, while a voltage of the opposite sign is produced, .presupposing energy has been bled from the crystals by a spark discharge to ground, by crystals 10 and 12 during the second half revolution of shaft 38, reaching a peak value when relaxation is at a maximum. The system is so designed that the peak volt-ages produced by the piezoelectric element near the maximum compression and relaxation thereof is greater than that required to fire spark plug 52, thereby being capable of passing gap 58. It will be apparent that the peak voltages must be applied to spark-plug 52 to fire it at the proper time. For example, in the case of a singlecylinder 4-cycle engine, shaft 38 must rotate at, or at one-half, the speed of the camshaft. Should shaft 38 rotate at the camshaft speed, it will make one revolution during each engine cycle, thereby resulting in a peak voltage of each sign being produced by the piezoelectric element during each engine cycle. The two peak voltages will cause spark-plug 52 to fire twice during each engine cycle, one at the usual time immediately prior to the piston power stroke, and again just before the downward intake stroke. Even though the latter firing will not adversely aifect the engine operation, it can be avoided, if desired, by gearing-down the speed of shaft 38 so that it rotates at one-half the camshaft speed.

The degree of pressure applied to piezoelectric crystals 10 and 12 by lever 32 can be varied by adjustment of setscrew 44 to cause the potential created by the piezoelectric element to increase to a level suflicient to pass gap- 58'at a time earlier or later during the rotation of eccentric 36. This provides a moderate degree of variability in timing, but large adjustments in timing are achieved by rotating eccentric 36 relative to the engine camshaft.

FIGURE 2 illustrates a spark plug which can be used in the embodiment of this invention in which the gap is in the spark plug rather than in the high-tension wiring be-.

tween the piezoelectric element and the spark plug. Referring to FIGURE 2, the numeral '70 represents an insulator composed of the usual insulating material now commonly used in spark-plugs, such as a ceramic. Insulator 70 can be of any desired shape or design as long as it functions to efficiently insulate the electrodes (yet to be described) of the spark plug from each other. Insulator 70 is held within lower body portion 72 by the action of upper bodyportion 74, which is threaded to fit within lower body portion 72 and adapted to press upon heat-resistant seal-rings 76 to fornra gas-tight seal about the enlarged center portion of insulator 70. Lower body portion 72 and upper body portion 74 are made of an electrically conductive material such as steel or brass. The central electrode is composed of a fixed electrode 78 within the lower part of insulator 70 and a movable electrode 80 within the upper part of insulator 70. Fixed electrode 78 is tightly sealed within insulator 70 in gas-tight relationship therewith and extends outof the lower-most portion of insulator 70. Movable electrode 80, which is spaced from' fixed electrode 78 to form gap 82, has threaded portion 84. Within the top center portion of insulator 70 is bushing 86 which has a central threaded :portion to match threaded portion 84 of movable electrode 80. Movable electrode 80 is, therefore, free to revolve within insulator 70, and by turning knurled thumb wheel 88 it can be raised or lowered therein to adjust the size of gap 82. If so desired, a graduated scale can be scribed on the portion of movable electrode 80 extending out of bushing 86 so that the mark even with the top of bushing 86 indicates the size of gap 82. Movable electrode 80 may be fitted with a lock-nut (not shown) to insure retention of the electrode, in any desired position and prevent vertical displacement due to engine vibration. Nut 90 is provided at the upper -end of electrode 80 for securing the wire leading to the piezoelectric element. Lower body portion 72 has electrode 92 which is in spaced sparking relation ship to fixed electrode 78. Electrode 92 may be an integral part of lower body portion '72 or may, as shown in the drawing,,be a separate piece tightly fitted in a recess of lower body portion 72.

'In'order to demonstrate how the gap in the high-tension wiring, or the auxiliary gap spark plug, functions to replace the timing switch of conventional piezoelectric ignition system, reference is made to FIGURES 3 and 4. In FIGURE 3, which is a schematic diagram of a piezoelectric ignition system embodying the instant invention, numeral 100 represents the piezoelectric generator which is fundamentally a special form of an electrical capacitor that can be charged by mechanical strain. Piezoelectric generator 100 is coupled to high-tension lead wire 102 by physical gap 104. Such a coupling gap which is often, but not always, used whether the ignition system be electromagnetic or piezoelectric serves to isolate the sparkenergy source from possible electrical leakage at the hightension line. The high-tension line has an inherent natural capacitance, represented figuratively by capacitor 106. The auxiliary gap of this invention 108 is shown in the hightension line which operatively connects piezoelectric generator to spark plug 110, but it can be incorporated within spark plug 110 as a part of the central electrode assembly. The series circuit is completed since both piezoelectric generator 100 and spark plug 110 are grounded to the engine assembly.

FIGURE 4 illustrates typical voltage curves as a function of time (crankshaft rotation) for piezoelectric ignition systems for four-cycle engines in which the piezoelectric generator is alternatelycompressed and relaxed by an eccentric (numeral 36 in FIGURE 1) rotating at the speed of the crankshaft (curve 120), and at the speed of the camshaft (curve 122). The solid portions of the curves represent the available spark-plug voltage, with the high tension lead connected to the spark plug, and the dotted portions of the curves represent the voltage which would be produced by the piezoelectric generator if the generator were not discharged until the peak voltages were reached.

In the case of a piezoelectric ignition system without either the conventional switch or the gap of this invention and where the eccentric is rotated at the speed of the crankshaft, the voltage rises along path AB of curve 120 until the voltage required to fire the spark plug is reached. This may be 6,000 volts for a spark plug in good condition, as shown at C, or 12,000 volts or more for a worn spark plug as shown at B. Depending upon the growth of the spark gap alone, timing may vary as much as 40. Other factors have been hereinbefore discussed. Changes in the spark-plug shunt resistance will actually distort the shape of the voltage build-up path, resulting in even greater possible variations in spark timing. Additionally, firing of the spark plug occurs at points D, E, and F. These are usually inconsequential at D and E, clearing the expansion and exhaust strokes, respectively, but would be disastrous at F, during the intake stroke. The only way they can be eliminated without using a timing switch is to have the eccentric rotate at the speed of the camshaft.

In the case of a conventional, switch-timed system in which the eccentric is rotated at the speed of the crankshaft, the voltage rises along path AB of curve on the compression stroke. When the end of the path is reached at B, the switch closes the circuit and the spark plug fires. The switch is designed to eliminate the firing of the spark plug at points D and F since a firing during the intake stroke would be detrimental.

The last case to be considered involves the embodiment of this invention where the piezoelectric element is compressed and relaxed once during the engine cycle, that is, the eccentric is rotated at the speed of the camshaft. Referring to curve 122, the voltage rise curve starts at G and builds up to H where the dielectric strength of the gap is exceeded and the spark plug then fires at the properly timed position. It can be seen'that the spark timing depends only upon the natural voltage rise characteristic of the generator, which in turn depends on the eccentric timing and the spacing of the gap of this invention. Changes in the spark plug condition cannot affect it, since the spark plug condition does not aifect the dielectric strength of the gap of this invention. The spark plug will also fire at I, but firing at this time is inconsequential.

Although this invention has been described in relation to specific embodiments, it is apparent that obvious modifications can be made by one skilled in the art without departing from the intended scope of this invention. For example, the piezoelectric ignition system of this invention may be used with multi-cylinder engines, and in 2-cycle as well as 4-cycle engines. Other arrangements of the piezoelectric element, and the mechanism for applying stress thereto, can be used.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an ignition system for a reciprocating internalcombustion engine having a spark plug, a piezoelectric element having opposite electrodes, means for gradually compressing and relaxing said piezoelectric element in timed relation to the engine cycle, and electric-circuit means for operatively connecting said piezoelectric element and the spark gap of said spark plug in series circuit, the improvement which comprises providing in said circuit means between one of the electrodes of said piezoelectric element and said spark gap a timing gap of about 0150-0188 inch being adapted to permit the passage of voltage required to fire the spark plug at a predetermined time in the engine cycle, said timing gap being the sole control of the spark plug firing in said circuit means and being adjustable before operation of the system and providing a constant gap during operation of the system.

2. An ignition system in accordance with claim 1 in which one of the electrodes of said piezoelectric element and one of the electrodes of said spark plug are grounded to the engine, and said timing gap is provided in the circuit means which operatively connects the ungrounded electrodes of said piezoelectric element and said spark gap.

3. An ignition system in accordance with claim 2 in which said circuit means between the ungrounded electrode of said piezoelectric element and said spark gap includes two timing electrodes in spaced relationship, one of said timing electrodes being operatively connected with the ungrounded electrode of said piezoelectric element and the other of said timing electrodes being operatively connected with the ungrounded electrode of said spark plug.

4. An ignition system in accordance with claim 3 in which said timing electrodes are enclosed within a housing of an electrically non-eoncluctive material.

said housing and the other of said timing electrodes is movable within said housing, and said housing includes means for moving said movable electrode toward and away from said stationary electrode.

7. An ignition system in accordance with claim 6 in which said movable electrode has an end in spaced relationship to said stationary electrode, a thumb wheel at the opposite end thereof extending out of said housing, and a screw-thread between its ends engaging a threaded member of said housing, 'and said movable electrode is movedtoward and away from said stationary electrode by means of said thumb wheel. I

8. An ignition system in accordance with claim 3 in which said gap .is within said spark plug.

9. An ignition system in accordance with claim 8 in which saidspark plug is comprised of a body of an electrically non-conductive material carrying two electrodes, one of said spark plug electrodes being a circumferential electrode surrounding said body and-electrically grounded to said engine, and the other of said spark plug electrodes being a centrally mounted electrode having two portions in spaced, end-to-end relationship to each other, the space between said portions forming said timing gap,-one of said portions having an active'end juxtaposed in sparking relationship with an .active portion of said-circumferential electrode, and the other portion being operatively con- .nected by electric-circuit means to the ungrounded elec trode of said piezoelectric element.

10. An ignition system in accordance with claim .9 in which said spark plug includes means for varying the space between the portions of said centrally mounted electrode While said spark gap is kept constant.

11. An ignition system in accordance with claim (10 in which the portion of said centrally mounted electrode,

having an active end juxtaposed in sparking relationship with an active portion of said circumferential electrode,

is stationary within said body, the other portion is .slidably mounted within saidbody, and said spark plug includes means for moving said slidably mounted portion toward and away from said stationary portion.

12. An ignition system in accordance with claim 11 in which the end of said slidably mounted portion of said central electrode remote from the end in spaced relationship to said fixed portion extends out of said body, said slidably mounted portion having a thumb wheel at the extendedend thereof and a screw thread between its ends, said screw thread engaging a threaded bushing secured within said body, and said slidably mounted portion is moved toward and away from :said stationary electrode by means of said thumbwheel.

References Cited by the Examiner UNITED STATES PATENTS 2,544,477 3/1951 West 3l5- 18() 2,674,237 4/1954 Peters "3 l5180 X 2,717,916 9/1955 Harkness 315.55 X 3,043,980 7/1962 Zalesak .3 l3--.124

JOHN W. HUCKERT, Primary Examiner.

JAMES D. KALLAM, Examiner. 

1. IN AN IGNITION SYSTEM FOR A RECIPROCATING INTERNALCOMBUSTION ENGINE HAVING A SPARK PLUG, A PIEZOELECTRIC ELEMENT HAVING OPPOSITE ELECTRODES, MEANS FOR GRADUALLY COMPRESSING AND RELAXING SAID PIEZOELECTRIC ELEMENT IN TIMED RELATION TO THE ENGINE CYCLE, AND ELECTRIC-CIRCUIT MEANS FOR OPERATIVELY CONNECTING SAID PIEZOELECTRIC ELEMENT AND THE SPARK GAP OF SAID SPARK PLUG IS SERIES CIRCUIT, THE IMPROVEMENT WHICH COMPRISES PROVIDING IN SAID CIRCUIT MEANS BETWEEN ONE OF THE ELECTRODES OF SAID PIEZOELECTRIC ELEMENT AND SAID SPARK GAP A TIMING THE PASSAGE OF 0.150-0.188 INCH BEING ADAPTED TO PERMIT THE PASSAGE OF VOLTAGE REQUIRED TO FIRE THE SPARK PLUG AT A PREDETERMINED TIME IN THE ENGINE CYCLE, SAID TIMING GAP BEING THE SOLE CONTROL OF THE SPARK PLUG FIRING IN SAID CIRCUIT MEANS AND BEING ADJUSTABLE BEFORE OPERATION OF THE SYSTEM AND PROVIDING A CONSTANT GAP DURING OPERATION OF THE SYSTEM. 